Photosensitivity in optical fibers: detection, characterization, and application to the fabrication of in-core fiber index gratings

The irradiation of optical fibers by intense optical fields from visible or ultraviolet laser sources creates color centers within the glass fiber. Such laser light irradiation usually has the detrimental effect of increasing the transmission loss of the optical fibers particularly in the visible spectral region. A concomitant effect of the light irradiation is that the refractive index of the glass is permanently changed even at wavelengths much longer than the wavelength of the irradiating light. This latter effect termed fiber photosensitivity is beneficial in that it provides a versatile means for fabricating periodic index gratings in the cores of optical fibers. Since the physical processes underlying fiber photosensitivity are not well understood, a purpose of this paper is to demonstrate the importance of the phenomena in order to stimulate further research on the origin of the effect and developing new photosensitive fiber materials. The paper reviews briefly the phenomena of photosensitivity in glass fibers with germanium dopant in the fiber core. The methods used for detecting and characterizing fiber photosensitivity are applied to a new photosensitive fiber - Eu²⁺:Al₂O₃- doped core fiber. This fiber was manufactured using a low-vapor-pressure aerosol-delivery MCVD method developed at Brown University and is the first fiber reported that is free of germanium dopant and exhibits fiber photosensitivity.